This invention is related to the field of offshore oil and gas production systems. In particular, the invention provides an improved choke system for subsea well completions.
In the field of offshore oil and gas production it is frequently desirable to use subsea well completions for oil and gas production operations. Such subsea well completions are particularly desirable in water where it would be uneconomical to install an offshore platform.
Several wells will generally be drilled in one area and the production from these wells will flow through a common subsea line to, for example, a central onshore processing plant or offshore platform. The pressure of fluids produced from various wells can vary significantly. Therefore, it is necessary to regulate the pressure which the higher pressure wells exert in the central flowline in order to permit lower pressure wells to flow. System which performs this function is known as a "choke" system. One such system is described in U.S. Pat. No. 4,589,493.
Prior art systems are generally configured as shown in FIG. 1. Such systems generally comprise a wellhead connector 1. Extending from the wellhead connector are a tubing connecting line 2 and an annulus connecting line 3. The tubing line has an upper end connected to a production lower master valve 4, whch is typically a gate valve and is used as a primary shutoff valve on the tubing string. Above and connected to the production lower master valve is the production upper master valve 5 which is used as a backup valve for shutting off the tubing. Above the upper master valve the tubing line divides. The upper portion is isolated by production swab valve 6 which is used for access to the well vertically in, for example, wireline operations.
Production wing line 7 is provided with a production wing valve 8, which can be used, for example, to isolate the well from the flowline 10 during well servicing operations. Downstream of the production wing valve, pressure control valve 9 is installed. Pressure control valve (PCV) 9, otherwise known as a choke, is used to regulate the downstream pressure of the flowing oil at an appropriate level for the flowline 10.
Annulus connecting line 3 is similarly provided with annular master valve 11, annular swab valve 12, and annular wing valve 13. Tree cap 14 is installed above the swab valves 6 and 12 to protect these lines from debris, sea growth, etc. Blind flanges 24, or the equivalent, are installed on the tree cap.
Such designs suffer several limitations, however. The pressure control (choke) valve in such installations is subjected to extreme wear and frequently requires replacement. This can be complicated, time consuming and costly in traditional well systems because the pressure control valve is integrated into the wellhead assembly. As a result, it is necessary to either remove the entire tree or have specialized tools for removal of the PCV. Further, if the PCV is adapted to be removed independently, it is necessary to install an additional block valve between PCV and the flowline so that the flowline may be isolated. Further, pigging of the flowlines and disposal of well test fluids can be complicated.
It is desirable, therefore, to devise an improved well choke system.